This invention relates to a pump with improved cold temperature performance for use in controlled vehicle braking systems, such as adaptive braking systems, traction control systems, and vehicle stability enhancement systems.
Controlled braking systems, such as adaptive braking systems, traction control systems, and vehicle stability enhancement systems, use a pump to force brake fluid to the vehicle brakes during such controlled operation. The pump must be self priming and must be able to force brake fluid, even cold viscous brake fluid, to one or more of the vehicle brakes in a relatively short time period. For example, a driver performing a quick steering maneuver which causes the vehicle to oversteer will require a counter-braking moment on the opposite front wheel to occur almost instantaneously in order to correct the skid condition. Accordingly, the pump must quickly extract brake fluid from the reservoir and force it to the appropriate brake under increasing pressure loads. The problem of quick response is particularly acute during cold weather operation, where the viscosity of the brake fluid places severe limitations on pump performance. Accordingly, to enhance cold weather performance, it is desirable to minimize internal restrictions within the pump, provide a pump having a relatively high compression ratio, and limit the travel of the pump inlet poppet.
According to the present invention, internal flow restrictions of the pump are minimized because the poppet return spring is placed in a separate cavity completely out of the flow path through the pump. The spring is a simple compression spring instead of the complex barrel spring used in prior art designs. Any viscous drag created by the spring and separate spring cavity is minimized by incorporating a slotted head on the poppet stem, in addition to grooves on the stem that permit fluid displacement between the spring cavity and the flow path through the pump. The spring cavity is closed by a wear button which extends from the piston and is engaged by an eccentric bearing to drive the pump piston, thereby enabling the rest of the assembly to be made of a softer steel. Poppet travel is limited by a step on the poppet stem or separate poppet retainer which contacts a corresponding face on the housing. Accordingly, the problem of the poppet continuing to open more and more at cold temperatures in response to increased brake fluid viscosity and resulting flow forces on the poppet is eliminated.